Power supplies for electronic equipment often need to provide more than one possible source of power to a system load, for reasons of redundancy. The multiple power sources might have the same power characteristics or they might be different from each other. For example, different power sources might be configured to provide power at different voltages. Where the power sources are different, or at least where one is typically preferred over another, the preferred power source is typically designated as the primary power source and the other is designated as the secondary power source.
When both a primary and secondary power source are required, a power supply should be designed so that when the primary source fails, the secondary source will immediately take over without an interruption in the operation of the equipment being powered. This is especially true in military applications, in which the equipment is required to conform to a demanding set of performance specifications.
FIG. 1 in the prior art depicts a block diagram of a redundant power supply, in which primary power source 101 and secondary power source 102 provide power to system load 110. Load 110 comprises the equipment that is being powered. Sources 101 and 102 are connected to load 110 by means of a relatively common technique called “diode-ORing.” The two power supplies are connected to load 110 through associated ORing-diodes 103 and 104, respectively, wherein source 101 provides voltage V1 and source 102 provides voltage V2. With the two power sources diode-ORed together, the power source with the larger output voltage will establish the voltage that is delivered to load 110 and which is present at node 105. By connecting the power sources in this way, if one power source fails, the other source will take up load 110 with little or no interruption in the power that is provided.
As is the case with the prior-art system depicted in FIG. 1, the primary power source, in addition to providing power to load 110 whenever possible, can also be used to charge the secondary power source, which is a battery in this case. When the primary power source is interrupted, because the battery-based secondary power source is kept charged, it can take over for the primary source.
There are operational scenarios, however, in which the secondary source's output voltage, V2, is higher than the primary source's output voltage, V1. For example, the primary power source might be designed to deliver 28 Volts DC at V1, and the secondary power source might be designed to deliver 33.6 Volts DC at V2, as is the case in certain military applications. In this case, the ORing diodes will select V2 as the voltage to be delivered to the load, as provided by the secondary power source. Ordinarily, this might be acceptable, especially if the battery of the secondary source is a more reliable source of power than the primary source. However, a mode of operation in which the secondary power source is normally selected might not be either desirable or sustainable—particularly, for example, if the lower-voltage primary source is being used to charge the higher-voltage secondary source.
It is, therefore, desirable to have the secondary power source charging and available as a backup to the primary power source during periods of low-power demand and also to have the primary power source available as a backup for the secondary power source during periods of high-power demand. To achieve this, what is needed is a power supply system without some of the disadvantages in the prior art.